Many apparatus that need information storage rely on some sort of storage device. A typical storage device is a hard disc drive, commonly known as a hard drive. Part of the evolution of such a device is increased storage capacity, smaller size and faster data availability. Inherent in that evolution is the increase of unit linear or areal density. For example, a hard drive contains at least one storage medium that saves information in concentric tracks. As the drives evolve, there is an increase in the tracks per inch (TPI) of the medium. Decreased tolerances are required for the mechanisms that read and write to those tracks.
Manufacturing processes have the tracks defined—prior to data being written and read—by a servo track writer. This servo track writer writes servo information to the storage medium. In one case, the track writer is entirely separate from the disc drive. This is known as multi-disc write (MDW). This servo information will be used by the disc drive to read and write information to the storage medium. Thus, the servo information is prewritten to the storage medium to be later used to define each track.
One unwelcome side effect of media with prewritten servo information is an increase in rotational eccentricity between the servo information and the axis of rotation of the disc drive. FIG. 1 illustrates this effect. A disc 100 centered at an axis point 110 has diametrically-opposed circumferential points 120 and 130. Assume that servo information is written at the circumference of disc 100 so that it includes points 120 and 130. Ideally, when disc 100 is rotated about axis point 110, all points along the circumference of disc 100 will pass through the same position as point 120. Axis 110 is the rotational axis of disc 100 in the servo writer.
However, when disc 100 is rotated about an axis point 140, which is offset from axis point 110, eccentricity is introduced. This occurs, for example, when disc 100 is placed in a disc drive. The axis 140 is the rotational axis of disc 100 in the disc drive. As a result, point 120 will follow a path signified by dashed circumference 150. Likewise, point 130 will follow a path signified by dashed circumference 160 and pass through point 170. If a read/write head of the disc drive were positioned over point 120 at the start of rotation about axis 140, then it would have to move to point 170 to stay on the center of the track. This movement, referred to as eccentricity, is indicated by a position error signal 200 shown in FIG. 2A. The frequency of this signal would be the rotational rate of disc 100.
Such eccentricity can be compensated for to a certain degree depending on its severity. Typically, a closed-loop servo system will obtain the eccentricity information from a position error signal (PES). In response, the system will provide a control signal that compensates for the eccentricity. However, such a system is constrained by the amount of eccentricity that it can compensate. Thus, when the eccentricity is large enough, this servo system is not effective.
The eccentricity described with reference to FIG. 1 also creates a problem with time-to-ready, an important disc drive metric. In particular, upon spin-up of the hard drive, the first harmonic (or 1f) coefficients for adaptive feedforward compensation (AFC) are initially set to zero. These coefficients are applied to sine and cosine values that are injected into a servo loop to compensate for the eccentricity. However, high eccentricity—which affects 1f coefficients—can potentially take many revolutions for the AFC coefficients to converge. These many revolutions used by the actuator to converge on track will ultimately have a negative impact on time-to-ready.